Method and system for icosaborane implantation

ABSTRACT

A method of implanting ionized icosaborane (B 20 H X ), triantaborane (B 30 H X ), and sarantaborane (B 40 H X ) into a workpiece is provided, comprising the steps of (i) vaporizing and ionizing decaborane in an ion source ( 50 ) to create a plasma; (ii) extracting ionized icosaborane, triantaborane, and sarantaborane (collectively “higher order boranes”) within the plasma through a source aperture ( 126 ) to form an ion beam; (iii) mass analyzing the ion beam with a mass analysis magnet ( 127 ) to permit ionized icosaborane (B 20 H X   + ) or one of the other higher order boranes to pass therethrough; and (iv) implanting the ionized icosaborane (B 20 H X   + ) or one of the other higher order boranes into a workpiece. The step of vaporizing and ionizing the decaborane comprises the substeps of (i) vaporizing decaborane in a vaporizer ( 51 ) and (ii) ionizing the vaporized decaborane in an ionizer ( 53 ).

RELATED PATENT AND PATENT APPLICATION

[0001] The following U.S. Patent and patent application, commonlyassigned to the assignee of the present invention, are incorporated byreference herein as if they had been fully set forth: U.S. Pat. No.6,107,634 to Horsky entitled DECABORANE VAPORIZER, and U.S. patentapplication Ser. No. 09/416,159 filed Oct. 11, 1999, entitled DECABORANEION SOURCE.

FIELD OF THE INVENTION

[0002] The present invention relates generally to semiconductorimplantation and more specifically to a method and system for implantingicosaborane, triantaborane, and sarantaborane ions into semiconductors.

BACKGROUND OF THE INVENTION

[0003] Conventional ion implantation systems, used for doping workpiecessuch as semiconductors, include an ion source that ionizes a desireddopant element which is then accelerated to form an ion beam ofprescribed energy. The ion beam is directed at the surface of theworkpiece to implant the workpiece with the dopant element. Theenergetic ions of the ion beam penetrate the surface of the workpiece sothat they are embedded into the crystalline lattice of the workpiecematerial to form a region of desired conductivity. The implantationprocess is typically performed in a high-vacuum process chamber whichprevents dispersion of the ion beam by collisions with residual gasmolecules and which minimizes the risk of contamination of the workpieceby airborne particulates.

[0004] Ion dose and energy are the two most important variables used todefine an implant step for a particular species. Ion dose relates to theconcentration of implanted ions for a given semiconductor material.Typically, high current implanters (generally greater than 10 milliamps(mA) ion beam current) are used for high dose implants, while mediumcurrent implanters (generally capable up to about 1 mA beam current) areused for lower dose applications.

[0005] Ion energy is used to control junction depth in semiconductordevices. The energy of the ions that make up the ion beam determines thedegree of depth of the implanted ions. High energy processes such asthose used to form retrograde wells in semiconductor devices requireimplants of up to a few million electron-volts (MeV), while shallowjunctions may only demand energies below 1 thousand electron-volts(keV), and ultra-shallow junctions may require energies as low as 250electron-volts (eV).

[0006] The continuing trend to smaller and smaller semiconductor devicesrequires implanters with ion sources that continue to deliver higherbeam currents at lower energies. The higher beam current provides thenecessary dosage levels, while the lower energy levels permit shallowimplants. Source/drain junctions in complementarymetal-oxide-semiconductor (CMOS) devices, for example, require such highcurrent, low energy applications.

[0007] Conventional ion sources utilize an ionizable dopant gas that isobtained either directly from a source of a compressed gas or indirectlyfrom a vaporized solid. Typical source elements are boron (B),phosphorous (P), gallium (Ga), indium (In), antimony (Sb), and arsenic(As). Most of these source elements are commonly used in both solid andgaseous form, except boron, which is almost exclusively provided ingaseous form, e.g., as boron trifluoride (BF₃), or as a compound insolid (powder) form as decaborane (B₁₀H₁₄).

[0008] Decaborane (B₁₀H₁₄) could be an excellent source of feed materialfor boron implants because each decaborane molecule (B₁₀H₁₄) whenvaporized and ionized can provide a molecular ion comprised of ten boronatoms. Such a source is especially suitable for high dose/low energyimplant processes used to create shallow junctions, because a moleculardecaborane ion beam can implant ten times the boron dose per unit ofcurrent as can a monatomic boron ion beam. In addition, because thedecaborane molecule breaks up into ten individual boron atoms of roughlyone-tenth the original beam energy at the workpiece surface, the beamcan be transported at ten times the energy of a dose-equivalentmonatomic boron ion beam. (The individual boron atoms of a singlycharged decaborane molecule (B₁₀H_(X) ⁺) of 10 identical boron atomsaccelerated with a voltage V each have an energy of eV/10, and thus theion beam can be extracted at 10 times the required energy). This featureenables the molecular ion beam to avoid the transmission losses that aretypically brought about by low-energy ion beam transport.

[0009] Recent process and ion source improvements have enabled thegeneration of ion beam currents that might prove in the future to besufficient for production applications of decaborane implants. Keys tosuch improvements are ion source cooling mechanisms that preventdissociation of the decaborane molecule and fragmentation of the desiredparent molecular ion (B₁₀H_(X) ⁺) into borane fragments and elementalboron. In addition, in known decaborane ion sources, such as that shownin U.S. Pat. No. 6,107,634, a low-density plasma is maintained toprevent the plasma itself from causing such dissociation andfragmentation.

[0010] As stated above, future ultra shallow junctions in semiconductorswill likely require boron implants with implant energies as low as 250eV. At such low energies, ion beam current densities will necessarilydecrease. Using even state-of-the art decaborane implant technology,semiconductor implant throughput will decrease unless implant doses canbe increased for the same level of ion beam current. In addition, itwill be desirable to increase the ion beam energy transport levelswithout increasing the energy levels of the individual boron atomsimplanted. Accordingly, these are objects of the present invention.

SUMMARY OF THE INVENTION

[0011] A method of implanting ionized icosaborane (B₂₀H_(X)),triantaborane (B₃₀H_(X)), and sarantaborane (B₄₀H_(X)) into a workpieceis provided, comprising the steps of (i) vaporizing and ionizingdecaborane in an ion source (50) to create a plasma; (ii) extractingionized icosaborane, triantaborane, and sarantaborane (collectively“higher order boranes”) within the plasma through a source aperture(126) to form an ion beam; (iii) mass analyzing the ion beam with a massanalysis magnet (127) to permit ionized icosaborane (B₂₀H_(X) ⁺) or oneof the other higher order boranes to pass therethrough; and (iv)implanting the ionized icosaborane (B₂₀H_(X) ⁺) or one of the otherhigher order boranes into a workpiece. The step of vaporizing andionizing the decaborane comprises the substeps of (i) vaporizingdecaborane in a vaporizer (51) and (ii) ionizing the vaporizeddecaborane in an ionizer (53).

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a schematic, partially cross sectional view of a firstembodiment of an ion source for an ion implanter constructed accordingto the principles of the present invention;

[0013]FIG. 2 is a cross sectional view of a connecting tube of analternative embodiment of the ion source of FIG. 1, taken along thelines 2-2;

[0014]FIG. 3 is a partially cross sectional view of the ionizer portionof the ion source of FIG. 1; and

[0015]FIGS. 4 and 5, taken together, are a plot of current versus atomicmass unit for an ion beam obtained using the ion source of FIG. 1,showing the presence of icosaborane, triantaborane, and sarantaboranecomponents.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

[0016] Referring now to FIGS. 1-3 of the drawings, and initially to FIG.1, an ion source 50 comprising a vaporizer 51 and an ionizer 53 areshown, constructed according to the present invention. The vaporizer 51comprises a non-reactive, thermally conductive sublimator or crucible52, a heating medium reservoir 54, a heating medium pump 55, atemperature controller 56, and a mass flow controller 60. Ionizer 53 isshown in more detail in FIG. 3. The crucible 52 is located remotely fromthe ionizer 53 and connected thereto by a feed tube 62, constructed ofquartz or stainless steel. In the disclosed embodiment, the feed tube 62is surrounded by an outer single-chamber annular sheath 90 alongsubstantially the entire length thereof.

[0017] The crucible 52 provides a container 64 enclosing a cavity 66 forcontaining a source material 68. The container is preferably made of asuitable non-reactive (inert) material such as stainless steel,graphite, quartz or boron nitride and which is capable of holding asufficient amount of source material such as decaborane (B₁₀H₁₄).

[0018] The decaborane is vaporized through a process of sublimation byheating the walls of the container 64 with a heating medium 70 containedin reservoir 54. Decaborane is typically available in fine powder formhaving a vapor pressure of 0.1 torr at room temperature and 19 torr at100° C. Completely vaporized decaborane exits the crucible 52 via feedtube 62 and enters mass flow controller 60, which controls the flow ofvapor, and thus meters the amount of vaporized decaborane which isprovided to the ionizer 53.

[0019] Alternatively, in a second embodiment of the invention, the feedtube 62 is provided in the form of a capillary tube and sheath 90 isprovided in the form of a coaxial dual-chamber sheath, comprising aninner sheath 90A surrounded by an outer sheath 90B (see FIG. 2). Theheating medium may be pumped into the inner sheath 90A (located adjacentthe capillary tube 62) and pumped out of the outer sheath 90B (locatedradially outward from the inner sheath 90A). In this second embodiment,the mass flow controller 60 is replaced with a heated shut-off valve(not shown) located at the feed tube/ionizer interface, and mass flow isincreased or decreased by directly changing the temperature of thereservoir 54.

[0020] The ionizer 53 is shown in more detail in FIG. 3. The ionizer 53comprises a generally cylindrical body 96 and a generally annular baseor mounting flange 98, both in the preferred embodiment constructed ofaluminum. The aluminum body 96 is cooled by means of entry coolingpassageway 100 fed by inlet 102 and exit cooling passageway 104 whichexits body 96 via outlet 106. The cooling medium may be water or anyother suitable fluid having high heat capacity. The entry and exitcooling passageways provide a continuous pathway by which water flowstherethrough to cool the ionizer body 96. Although only a fragmentedportion of the pathway is shown in phantom in FIG. 3, the pathway mayextend near and about the outer periphery of the body in any knownconfiguration to insure that the entire body is effectively cooled.

[0021] Cooling the body 96 insures that the ionization chamber 108resides at temperatures that will accommodate decaborane pressure withinthe ionization chamber that is sufficiently high. It has been found thatthe ionizer 53 should be maintained at a temperature low enough (lessthan 350° C. and preferably between 300° C. and 350° C.) to preventdissociation and fragmentation of the ionized decaborane molecule.

[0022] Referring back to FIG. 3, within the confines of the ionizer body96 are an extension of the feed tube 62, surrounded by annular sheath90, terminating at ionization chamber 108. Within the ionization chamberreside a hot cathode 110 and an anti-cathode or repeller 112. The hotcathode 110 comprises a heated tungsten filament 114 surrounded by amolybdenum cylinder 116 and capped by tungsten endcap 118. The heatedfilament 114 is energized via power feedthroughs 120 and 122 that passthrough and are electrically insulated from the aluminum body 96. Therepeller 112 is also electrically insulated from the body 96, via athermally conductive electrically insulating material (such as sapphire)which physically couples the repeller to the cooled ionization chamber108.

[0023] In operation, the vaporized decaborane powder is injected intothe ionization chamber via feed tube 62 at ionizer inlet 119. When thetungsten filament 114 is energized electrically by application of apotential difference across feedthroughs 120 and 122, the filament emitselectrons that accelerate toward and impact endcap 118. When the endcap118 is sufficiently heated by electron bombardment, it in turn emitselectrons into the ionization chamber 108 that strike the vaporized gasmolecules to create ions in the chamber.

[0024] A low-density ion plasma is thereby created, from which an ionbeam is extracted from the chamber through source aperture 126. Theplasma includes decaborane ions (B₁₀H_(X) ⁺), where X is an integer upto 14, icosaborane ions (B₂₀H_(X) ⁺), where X is an integer up to 28,triantaborane ions (B₃₀H_(X) ⁺), where X is an integer up to 42, andsarantaborane ions (B₄₀H_(X) ⁺), where X is an integer up to 56, all ofwhich are implantable into a workpiece. The term “icosaborane” isintended to include both B₂₀H_(X) molecules and/or a cluster of twodecaborane molecules. The term “triantaborane” is intended to includeboth B₃₀H_(X) molecules and/or a cluster of three decaborane molecules.The term “sarantaborane” is intended to include both B₄₀H_(X) moleculesand/or a cluster of four decaborane molecules. The extracted ion beam isthen mass analyzed by mass analysis magnet 127 to permit only ionshaving a prescribed charge-to-mass ratio to pass therethrough. The lowdensity of the decaborane/icosaborane plasma in chamber 108 is in partprovided by the relatively low arc discharge power maintained in thesource (about 5 watts (W) at 50 milliamps (mA)).

[0025] The constituency of the low-density plasma in ionizer 53 is shownin the graphs of FIGS. 4 and 5, which taken together show a plot ofcurrent versus atomic mass unit (AMU) for the components of an ion beamobtained using the ion source of FIG. 1. As shown in these Figures,individual definable peaks represent decaborane (B₁₀H_(X)) andicosaborane (B₂₀H_(X)) in FIG. 4, and B₃₀H_(X) and B₄₀H_(X) in FIG. 5.In FIG. 4, the decaborane peak is observed at 117 AMU, and theicosaborane peak is observed at 236 AMU. In FIG. 5, the B₃₀H_(X) peak isobserved at approximately 350 AMU and B₄₀H_(X) peak is observed atroughly 470 AMU.

[0026] Using the source 50 of FIG. 1 in an ion implanter, an entiredecaborane molecule (ten boron atoms) is implanted into the workpiece.The molecule breaks up at the workpiece surface such that the energy ofeach boron atom is roughly one-tenth the energy of the ten-boron groupof atoms (in the case of B₁₀H₁₄). Thus, the beam can be transported atten times the desired boron implantation energy, enabling very shallowimplants without significant beam transmission losses. In addition, at agiven beam current, each unit of current delivers ten times the dose tothe workpiece. Finally, because the charge per unit dose is one-tenththat of a monatomic beam implant, workpiece charging problems are muchless severe for a given dose rate.

[0027] With regard to the icosaborane (B₂₀H_(X)) component of the ionbeam that is extracted from the ion source through source aperture 126,it is believed that the icosaborane is being formed by the irradiationof decaborane with H or H₂ ions within the ionizer 53 of source 50.Under specific source conditions, decaborane vapor in the ionizer 53results in an adjacent sub-spectrum from mass 130 AMU to 240 AMU (seeFIG. 4) that appears to be similar to the decaborane spectrum. Thus, theion beam extracted through the ion source aperture 126 includes, inaddition to the fragmented decaborane spectrum, a fragmented decaborane“dimer” (also referred to herein as a dual cluster of moleculardecaborane or icosaborane (B₂₀H_(X))). Ionized icosaborane (B₂₀H_(X) ⁺)is implantable at energy per boron atom of below 500 eV, and even as lowas 250 eV, at boron particle currents as high as 1.6 millamp (mA).

[0028] Using the source 50 of FIG. 1 in an ion implanter, an entireicosaborane ion (twenty boron atoms) is implanted into the workpiece.The ion breaks up at the workpiece surface such that the energy of eachboron atom is roughly one-twentieth the energy of the twenty-boron groupof atoms (in the case of B₂₀H_(X)). Thus, the beam can be transported attwenty times the desired boron implantation energy, enabling veryshallow implants without significant beam transmission losses. Inaddition, at a given beam current, each unit of current delivers twentytimes the dose to the workpiece. Finally, because the charge per unitdose is one-twentieth that of a monatomic beam implant, workpiececharging problems are much less severe for a given dose rate.

[0029] The mass analysis magnet 127 can be adjusted, as is known in theart, to permit to pass therethrough only particles having acharge-to-mass ratio within a specific range. Accordingly, the massanalysis magnet 127 may be adjusted to permit either moleculardecaborane (B₁₀H₁₄) or icosaborane (B₂₀H_(X)) to pass therethrough.Still further, the magnet may be adjusted to permit passage therethroughof either of the other higher order boranes (B₃₀H_(X) and B₄₀H_(X)) inthe ion beam.

[0030] Electrons generated by cathode 110 which do not strike adecaborane molecule or one of the higher order boranes in the ionizationchamber to create a decaborane or icosaborane ion move toward therepeller 112, which deflects these electrons back toward the cathode.The repeller is preferably constructed of molybdenum and, like thecathode, is electrically insulated from the ionizer body 96. Walls 128of the ionization chamber 108 are maintained at local electrical groundpotential. The cathode 110, including endcap 118, is maintained at apotential of approximately 50 to 150 volts below the potential of thewalls 128. The filament 114 is maintained at a voltage approximatelybetween 200 and 600 volts below the potential of the endcap 118. Thelarge voltage difference between the filament 114 and the endcap 118imparts a high energy to the electrons emitted from the filament tosufficiently heat endcap 118 to thermionically emit electrons into theionization chamber 108.

[0031] The ion source 50 provides a control mechanism for controllingthe operating temperature of the crucible 52, as well as that of thefeed tube 62 through which vaporized decaborane passes on its way to andthrough the ionizer 53. The heating medium 70 is heated within thereservoir 54 by a resistive or similar heating element 80 and cooled bya heat exchanger. The temperature control means comprises a temperaturecontroller 56 which obtains as an input temperature feedback from thereservoir 54 via thermocouple 92, and outputs a control signal toheating element 80, as further described below, so that the heatingmedium 70 in the reservoir is heated to a suitable temperature.

[0032] The heating medium 70 comprises mineral oil or other suitablemedium (e.g., water) that provides a high heat capacity. The oil isheated to a temperature within the 20° C. to 250° C. range by theheating element 80 and circulated by pump 55 around the crucible 52 andthe feed tube 62 through sheath 90. The pump 55 is provided with aninlet and an outlet 82 and 84, respectively, and the reservoir 54 issimilarly provided with an inlet 86 and an outlet 88, respectively. Theflow pattern of the heating medium about the crucible 52 and the feedtube 62, although shown in a unidirectional clockwise pattern in FIG. 2,may be any pattern that provides reasonable circulation of the mediumabout the crucible 52 and the feed tube 62.

[0033] Referring back to FIG. 1, the crucible cavity 66 is pressurizedin order to facilitate material transfer of the vaporized (sublimated)decaborane from the crucible 52 to the ionization chamber 108 throughthe feed tube 62. As the pressure within cavity 66 is raised, the rateof material transfer correspondingly increases. The ionization chamberoperates at a near vacuum (about 1 millitorr), and thus, a pressuregradient exists along the entire length of the feed tube 62, from thecrucible 52 to the ionization chamber 108. The pressure of the crucibleis typically on the order of 1 torr.

[0034] By locating the crucible 52 remotely from the ionization chamber108, the material within crucible cavity 66 is thermally isolated,thereby providing a thermally stable environment unaffected by thetemperature in the ionization chamber. As such, the temperature of thecrucible cavity 66, in which the process of decaborane sublimationoccurs, may be controlled independently of the operating temperature ofthe ionization chamber 108 to a high degree of accuracy (within 1° C.).Also, by maintaining a constant temperature of the vaporized decaboraneduring transport to the ionization chamber via the heated feed tube 62,no condensation or thermal decomposition of the vapor occurs.

[0035] The temperature controller 56 controls the temperature of thecrucible 52 and the feed tube 62 by controlling the operation of theheating element 80 for the heating medium reservoir 70. Thermocouple 92senses the temperature of the reservoir 70 and sends temperaturefeedback signal 93 to the temperature controller 56. The temperaturecontroller responds to this input feedback signal in a known manner byoutputting control signal 94 to the reservoir heating element 80. Inthis manner, a uniform temperature is provided for all surfaces to whichthe solid phase decaborane and vaporized decaborane are exposed, up tothe location of the ionization chamber.

[0036] By controlling the circulation of the heating medium in thesystem (via pump 55) and the temperature of the heating medium (viaheating element 80), the ion source 50 can be controlled to an operatingtemperature of on the order of 20° C. to 250° C. (+/−1° C.). Precisetemperature control is more critical at the crucible, as compared to theend of the feed tube nearest the ionization chamber, to control thepressure of the crucible and thus the vapor flow rates out of thecrucible.

[0037] Because the plasma density using the inventive source is kept low(on the order 10¹⁰/cm³) to prevent dissociation of thedecaborane/icosaborane molecular structure, total extracted ion beamcurrent will be low when using a conventionally-sized source aperture.Assuming a comparable beam current density, the aperture 126 in theionizer 53 of the present invention is made large enough to insure anadequate ion beam current output. A 1 cm² (0.22 cm×4.5 cm) aperturepermits a beam current density of about 100 microamps per squarecentimeter (μA/cm²) at the workpiece, and up to (less than or equal to)1 mA/cm² of extracted beam current from the source. (The actual focusedbeam current delivered to the workpiece is only a fraction of the totalextracted beam current.) Aperture sizes of about 5 cm² are possible insome implanters, which would yield a B₁₀H_(X) ⁺ beam current of about500 μA at the workpiece. In ultra low energy (ULE) implanters, evenlarger aperture sizes (up to 13 cm²) are possible.

[0038] Accordingly, a preferred embodiment of an improved method andsystem for implanting decaborane or icosaborane has been described. Withthe foregoing description in mind, however, it is understood that thisdescription is made only by way of example, that the invention is notlimited to the particular embodiments described herein, and that variousrearrangements, modifications, and substitutions may be implemented withrespect to the foregoing description without departing from the scope ofthe invention as defined by the following claims and their equivalents.

1. A method of implanting ionized higher order boranes into a workpiece,comprising the steps of: (i) vaporizing and ionizing decaborane in anion source (50) to create a plasma; (ii) extracting ionized higher orderboranes within the plasma through a source aperture (126) to form an ionbeam; (iii) mass analyzing the ion beam with a mass analysis magnet(127) to permit a selected ionized higher order borane to passtherethrough; and (iv) implanting the selected ionized higher orderborane into a workpiece.
 2. The method of claim 1, wherein said step ofvaporizing and ionizing decaborane in an ion source (50) comprises thesubsteps of (i)(a) vaporizing decaborane in a vaporizer (51) and (i)(b)ionizing the vaporized decaborane in an ionizer (53).
 3. The method ofclaim 2, wherein said higher boranes include icosaborane (B₂₀H_(X)),triantaborane (B₃₀H_(X)), and sarantaborane (B₄₀H_(X)) and wherein theselected ionized higher order borane is ionized icosaborane (B₂₀H_(X)⁺).
 4. The method of claim 2, wherein said higher boranes includeicosaborane (B₂₀H_(X)), triantaborane (B₃₀H_(X)), and sarantaborane(B₄₀H_(X)), and wherein the selected ionized higher order borane isionized triantaborane (B₃₀H_(X) ⁺).
 5. The method of claim 2, whereinsaid higher boranes include icosaborane (B₂₀H_(X)), triantaborane(B₃₀H_(X)), and sarantaborane (B₄₀H_(X)), and wherein the selectedionized higher order borane is ionized sarantaborane (B₄₀H_(X) ⁺). 6.The method of claim 2, wherein said ionizer (53) is provided with anionization chamber (108), further comprising the step of activelycooling walls (128) of said ionization chamber (108) during the step ofionizing the vaporized decaborane.
 7. The method of claim 6, whereinsaid step of cooling said ionization chamber walls (128) maintains atemperature of said walls below 350° C. to prevent dissociation ofvaporized decaborane molecules.
 8. The method of claim 7, wherein saidtemperature of said walls is maintained between 300° C. and 350° C. 9.The method of claim 2, wherein said source aperture (126) is sized toprovide a focused ion beam current of between 100-500 microamps (μA) ata beam current density of <1 milliamp per square centimeter (mA/cm²).10. The method of claim 9, wherein said plasma has a density within saidionization chamber (108) on the order of 10¹⁰/cm³.